1. Field of the Invention
This invention relates to a developer for developing electrostatic latent images in electrophotography, electrostatic recording, electrostatic printing, and an image forming method. More particularly, it pertains to an electrophotographic developer which can charge strongly and uniformly negative charges to give images of high quality with little dependence on environment in the direct or indirect electrophotographic developing method.
2. Related Background Art
In the prior art, electrophotogrphic methods as disclosed in U.S. Pat. Nos. 2,297,691, 3,666,363 and 4,071,361 have been known. Generally speaking, by utilizing a photoconductive substance, forming an electrical latent image on a photosensitive member according to various means, and then developing said latent image with the use of a developing powder (hereinafter called toner) and, after optionally transferring the toner image onto a transfer material such as paper, fixing the image by heating, pressure or heating under pressure or by use of a solvent vapor to give a copied product. When having the step of transferring the toner image, there is ordinarily provided a step for removing the residual toner on the photosensitive member.
As the method for visualizing the electrical latent images with the use of a toner, there may be included the magnetic brush method as disclosed in U.S. Pat. No. 2,874,063, the cascade developing method as disclosed in U.S. Pat. No. 2,618,552 and the powder cloud method as disclosed in U.S. Pat. No. 2,221,776. As the method of employing magnetic toner, there may be included the magnedry method by use of an electroconductive toner as disclosed in U.S. Pat. No. 3,909,258, the method of employing dielectric polarization of toner particles, and the charge delivery method by disturbance of the toner. Further, there is the method in which development is effected by propelling toner particles toward latent images, as disclosed in U.S. Pat. Nos. 4,356,245 and 4,395,476.
In the toner applied for these methods, there have been used in the art fine powder containing a dye and/or pigment dispersed in a natural or synthetic resin. For example, particles finely pulverized to about 1 to 30.mu. of a colorant dispersed in a binder such as polystyrene have been used as the toner. As the magnetic toner, those containing magnetic particles such as magnetite or ferrite have been used. On the other hand, in the case of a system employing two-component developers, a mixture of a toner with carrier particles such as glass beads or iron powder has been used.
In the method of using such dry system developer, in order to form visible images of good quality on the latent image carrier, the developer is required to have high flowing characteristic and have uniform chargeability. For this purpose, it has been practiced in the art to add and mix silica fine powder in toner powder. However, since silica fine powder is itself hydrophilic, the developer added with this powder may cause agglomeration due to humidity in the air to be lowered in flowing characteristic, or in an extreme case, may lower chargeability of the developer due to mositure absorption by the silica. For this reason, it has been proposed to use silica fine powder subjected to hydrophobic treatment in U.S. Pat. Nos. 3,720,617, 3,819,367, 3,983,045 and U.K. Patent No. 1,402,010. More specifically, it is the method in which silicon dioxide fine particles (silica fine powder) are reacted with a silane coupling agent to make them hydrophobic by replacement of silanol groups on the surface of the silicon dioxide fine particles with other organic groups. As the silane coupling agent, there are exemplified dimethyldichlorosilane, trimethylalkoxysilane, hexamethyldislazane and the like.
However, these silica fine powder, although modified to hydrophobic in nature to some extent, the extent of hydrophobic modification is not yet sufficient, and when left to stand under highly humid condition, the developer may tend to be lowered in charging performance. In recent years, copying machines, laser printers of small size and low price are appearing in the market. Thus, the circumstances in which these devices are used are not limited to offices with relatively good environmental conditions adjusted by means of air conditioner, but also are open to use in homes in general. Under such environment, it is necessary to maintain good copying quality even when left to stand under highly humid condition for a long term, and in this respect the silica fine powder subjected to hydrophobic modification of the prior art has still possess the points to be improved.
In recent years, copying machines or laser printers of small size and low price for personal use have appeared, and in these small size machines, there has been used the cartridge system in which the photosensitive member, the devloping instrument and the cleaning device are integrally assembled from the maintenance free standpoint. Since this cartridge is made disposable, an inexpensive organic photoconductive member (OPC) has been used as the photosensitive member. Further, as the mode suitable for personal use, the copying machine and laser printer itself is required to be miniaturized, and for this purpose a photosensitive with a small drum diameter has been demanded. Also, for the cleaning device, a blade cleaning for which the device can be made simple has been employed. Similarly, as the developer, it is preferable to use magnetic one-component system developer which makes the structure of the developing instrument simpler.
In such magnetic toner, the polishing effect of toner itself is strong, and when a photosensitive member with low surface hardness such as OPC is used as the photosensitive member and cleaning to effect strong pressure contact against the photosensitive member such as blade cleaning system is performed, with the use of the toner externally added with silica fine powder treated with a silane coupling agent of the prior art, photosensitive member contamination such as white drop-out due to cutting of the photosensitive member surface or toner fusion, black dots or filming due to damaging of the photosensitive member is liable to occur, to give rise to image defects in an extreme case. For avoiding such phenomenon, there has been known in the prior art the method to add a lubricant (e.g. fatty acid metal salt such as zinc stearate) in the toner. However, most of these lubricants have strong polarity and, when attached on the photosensitive surface, may frequently cause the trouble of image flowing under highly humid condition, thus having points to be improved.
In the prior art, in a digital copying machine or printer, latent images are constituted of basic picture elements (hereinafter called dots), and halftone images, solid black images and solid white images constituted of dots. Accordingly, during development, developing due to the edge effect is predominant. The edge effect is a phenomenon in which concentration of electrical lines of force occur at the boundary portion between the exposed portion and nonexposed portion of a latent image, whereby the surface potential of the photosensitive member is apparently raised to increase the image density at the boundary portion. In the prior art, in the analog development, this phenomenon is not favorable, because the solid image becomes nonuniform (image density increased at the end portion).
In the digital image forming method, in which a latent image is expressed with picture elements of 50 to 150 .mu.m, since the portion receiving the edge effect is greater than the analog image in general, development with good line reproduction and high image density can be realized. The speciality of development of the edge portion resides in that unless the gradient of potential is great and the charging amount of developer or toner is sufficiently high, since the toner with greater charging amount is selectively used, the developer with low charging amount in the developing instrument is liable to reside in the machine, whereby deterioration will be readily caused after repeated copying of a large number of sheets. For this reason, it is important that the charging amount on the toner particles in the developer should be uniform.
This tendency poses frequently problems in deterioration of image during successive copying and narrowing of the line due to speciality of the edge phenomenon, particularly in such systems as laser printer, liquid crystal printer, etc., because of the primary output of letter images, among digital latent image systems.
In the prior art electrophotographic system, normal development has been primarily effected on the nonexposed portion. Recently, in the printer system in which image signals are expressed digitally, for elongation of life of the emitting body (semiconductor laser) to be used for developing exposure and improvement of image quality, it has been proposed to use the reversal developing system in which development is effected on the exposed portion with a toner of the same polarity as the latent image charges.
In the above reversal developing system, during developing, the toner is developed by the electric field at the site of the non-charge portion or the same polarity on the photosensitive member, and held on the photosensitive surface by the charges generated on the photosensitive surface through electrostatic induction of the toner having charges.
For the toner to be held stably at the reversal latent image position on the photosensitive member, it is necessary to increase the charging amount of the toner or developer which causes electrostatic induction.
In the reversal developing system, since the transfer material (plain paper or plastic sheet) is charged to the opposite polarity of the latent image charges on the photosensitive member during transfer, if the current contributing to transfer is increased, the winding phenomenon is liable to occur, in which transfer material and the photosensitive member are electrically adhered to each other.
For this reason, the transfer current has been limited to about half of the prior art, and in order to prevent lowering in transfer efficiency with low electrical field, the charging amount of the toner or developer is required to be made higher.
When a developer with low charging amount and broad charging amount distribution of toner particles group is supplied for a reversal developing system, during development, developability will be lowered to lower image density due to shortage in charging amount. Further, since the toner with good charging amount is preferentially consumed, in toner or developer with relatively lower in charging amount remain such on the developing sleeve, whereby image deterioration will occur by successive copying.
During transfer, due to shortage in charging amount, transfer efficiency is lowered to lower the image density, and also the toner with smaller charging amount can be restricted by the electrical field with difficulty, and therefore scattering of the toner will occur during transfer to cause lowering in image quality.
In any case, in the system having the development-transfer mechanism of the normal developing system of the prior art, although the influence may be small, shortage in charging amount of the developer becomes particularly the problem in the case of the reversal developing system. In the reversal development practiced in laser printer, due to smaller chages of electrostatic latent images on the image portion and greater charges of the background on the photosensitive member, the toner is carried on the background with greater charges on the photosensitive member if a toner with smaller charge amount exists. Prevention of this reversal fogging phenomenon has been the most important task in the reversal developing process.